Determination of phosphates bases on the creating of chemical compound with molybdenic heteropoly acids in acid environment. Water solution is not sufficiently intense color, so usually use reduced form of molybdenic heteropoly acid. This form is blue with bigger optical density. There are different reducing agents for molybdenic heteropoly acid such as ascorbic acid, sodium sulfite, stannum (II)chloride, hydroquinone. Most popular is stannum (II)chloride, because it gives the most intensive colouration.
The following substances hinder analysis: silicates(>50 mg/L), ions of iron (>1 mg/L), chromates (>2 g/L), sulfides (>3 g/L), nitrites, as well as greater amount of chlorides, arsenates.
PROCEDURE FOR THE DETERMINATION
Add to probe (50 mL) one milliliter of acid molybdate solution. After 5 minutes, add 0,1 mL of stannum (II) chloride. After 10 minutes, you can do photometric measuring (λ=690-720 nm) in cuvette with a width of 2 or 5 centimeters.
PREPARATION THE REAGENTS
1. The acid solution of molybdate
Dissolve 25g of ammonium molybdate ((NH4)6Mo7O24• 4H2O) in 600 mL of water. Add to mixture 340 mL a concentrated sulphuric acid while stirring. The solution must be cooled. Increase the volume of solution until 1 liter. You can use this solution after 48 hours. The solution to keep in the dark.
2. The solution of stannum (II) chloride
The solution of stannum (II) chloride must be freshly made. It must be prepared by dilution in 4 times from concentrated solution. The concentrated solution prepares dissolution to 2g stannum (II) chloride (SnCl2•2H2O) in 50 ml diluted acid (18 mL HCl in 50 mL of water). The got suspension must be kept in vessel with paraffin walls. I kept suspension in the ordinary chemical flask for 3 month. Nothing bad has not occurred!
3. The standard solution of phosphate
The standard solution of phosphate prepares from potassium hydrophosphate (KH2PO4). Salt should be dried up at temperature 100 0C within 2 hours. Dissolve 0,1433 g of potassium hydrophosphate in 1000 mL of water. This solution will contains 0,1 g/L of phosphate. Shelf life 3 months. Dilute solution to use for one day.
I recommend to keep the concentrated solution in refrigerator.
Attention! You must wash photometric cuvette in the alcohol after work, because molybdophosphoric acid is very sticky. Better alcohol is butyl alcohol.